IPA: /ˈstɑːrtlər/
KK: /ˈstɑrtlər/
A person or thing that causes someone to feel sudden shock or surprise.
The loud noise was a real startler for everyone in the room.
Startler is formed from "start" (from Old English *styrtan*, meaning to leap up or jump) and "-ler" (a suffix indicating a person who performs an action). The word "startler" refers to a person who causes someone to jump or leap, often in surprise.
Think of someone who makes you 'jump' ('start') — that's why a startler is someone who causes a start or surprise.